1. Field of the Invention
The invention concerns telecommunication networks and more particularly optical communication systems in which optical digital data is transmitted, routed and processed.
2. Description of the Prior Art
The information conveyed in such systems is usually binary data in the form of pulses timed at a particular modulation clock frequency. The amplitudes of the pulses represent the binary data. The pulses are initially in electrical form and are then converted into an optical signal by modulating the intensity (optical power) of an optical carrier.
Multiplexing techniques such as time-division multiplexing and/or wavelength-division multiplexing are used in order for the system to be able to convey a large number of optical signals exploiting the bandwidth of optical links.
In most optical communication systems using more than one wavelength it is generally desirable, at some points in the network at least, for more than one optical signal to have intensity modulation conforming to strictly defined phase relationships (to the nearest 2xcfx80).
This problem arises at nodes where a plurality of optical signals with different wavelengths from independent sources have to be perfectly synchronized before they are combined to form a wavelength-division multiplex, for example.
A similar problem arises in simultaneously regenerating streams of wavelength-division multiplexed solitons using a modulator. The streams of solitons must be resynchronized before they are fed into the modulator, which is controlled by a clock signal synchronized with those of the streams of solitons.
Another example is that of time-division multiplexing interfaces where a plurality of optical signals are interleaved temporally before they are combined to form the time-division multiplex.
One feasible solution is to recover the clocks associated with the various signals to be synchronized, comparing their phases and applying time-delays to the signals in accordance with the comparison results. However, this solution has the drawback of necessitating clock recovery more than once.
Accordingly, one aim of the invention is to propose a solution to the synchronization problem that avoids the aforementioned drawback. Another aim is to enable implementation not only of the optical part but also of the associated electronic circuits.
To be more precise, the invention consists in a method of resynchronizing optical signals in which a variable time-delay is applied to a first or a second input optical signal to supply a delayed optical signal phase-locked relative to the other input optical signal, said first and second input optical signals conveying information by intensity modulation of optical carrier waves having different optical frequencies f1 and f2, said method being characterized in that it consists in:
forming first and second measurement optical signals respectively representative of said delayed optical signal and said other input optical signal,
forming a combination signal by coupling said first and second measurement optical signals,
forming a mixed signal by injecting said combination signal into a non-linear optical device,
forming a filtered signal by filtering said mixed signal using a filter tuned to an optical frequency f3 equal to p.f1+q.f2, p and q being relative integers such that the frequency f3 is different from the frequencies f1 and f2 of the carrier waves,
forming a control electrical signal representative of the average value of the intensity of said filter signal, and
controlling said variable time-delay in accordance with said control signal.
Accordingly, the method exploits the property of non-linear optical media whereby intermodulation phenomena occur if two optical signals with different carrier frequencies f1, f2 are fed into the medium, for example. The medium then generates intermodulation signals at optical frequencies in the form p.f1+q.f2, where p and q are generally relative integers that of course make the preceding expression positive. The amplitude of the modulation of each signal is in particular representative of the difference between the phases of the signals. Experiments have shown that the average value of the intensity of a selected intermodulation signal is a good representation of the phase difference.
The method is executed partly in the optical domain and partly in the electrical domain, with the advantage that the processing in the electrical domain does not require any circuit operating at a high frequency, which considerably simplifies implementation.
The method as defined hereinabove can also be used to resynchronize more than two signals. For example, it is sufficient to apply the method to first two signals, which supplies two resynchronized output signals, and then to apply the method again to a third signal and to one of the first two resynchronized output signals, and so on.
In a preferred embodiment, said numbers p and q are such that the difference between said frequency f3 of the filter and one of the frequencies f1 or f2 of the carrier waves is equal to the difference between said carrier wave frequencies f1 and f2.
This assures that the frequency f3 is close to the frequencies f1 and f2, i.e. is in the usual range of optical transmission frequencies. Accordingly the optical components processing the mixed signal (filter, photodetector) can be of conventional types.
In accordance with another aspect of the invention, the method is further characterized in that said first and second measurement optical signals are respectively sampled from said delayed optical signal and from said other input optical signal and in that said combination signal is amplified optically before it is injected into a non-linear optical medium.
In accordance with another aspect of the invention said electrical control signal is obtained by electrical low-pass filtering of a converted signal supplied by a photodetector to which said filtered signal is applied.
The invention also consists in a device for resynchronizing optical signals for implementing the method defined hereinabove. The resynchronization device includes a variable time-delay device adapted to receive a first or a second input optical signal and adapted to supply a delayed optical signal phase-locked relative to the other input optical signal, said first and second input optical signals conveying information by modulating the intensity of optical carrier waves having different optical frequencies f1 and f2. The device is characterized in that it comprises:
sampling means for forming first and second measurement optical signals respectively representative of said delayed optical signal and said other input optical signal,
coupling means adapted to receive said first and second measurement optical signals and adapted to supply a combination signal,
a non-linear optical device adapted to receive said combination signal and adapted to supply a mixed signal,
a filter adapted to receive said mixed signal and adapted to supply a filtered signal, said filter being tuned to an optical frequency f3 equal to p.f1+q.f2, p and q being relative integers such that the frequency f3 is different from the frequencies f1 and f2 of the carrier waves,
a converter device for forming a control electrical signal representative of the average value of the intensity of said filtered signal, and
control means for controlling said variable time-delay device in accordance with said control signal.
Other aspects and advantages of the invention will become apparent in the remainder of the description with reference to the figures.